The specific aims of this study are-- 1) To test the hypothesis that aging is associated with decreased themic effect of feeding (TEF). This hypothesis will be tested in a study in which healthy young and elderly women are fed test meals containing four different levels of energy intake (0, 250, 500, 1000 kcals), and TEF is followed until a return to baseline to determine whether the dose-response relationship and/or the time course of TEF and substrate oxidation differs with age. Repeated measurements will be made on the young women due to the possibility that the stage of the menstrual cycle may influence TEF. 2) To determine the factors that may cause a decrease in TEF in the elderly by examinig associations between TEF, body fat content and distribution, fitness (VO2max), insulin sensitivity, substrate use, sympathetic nervous system activity, and thyroid hormone.